This study is a randomized, double-blind, multicenter, phase III clinical study to compare the clinical efficacy and safety of Serplulimab + chemotherapy+ concurrent radiotherapy vs chemotherapy+ concurrent radiotherapy in subjects with Limited-Stage Small Cell Lung Cancer.

Eligible subjects in this study will be randomized to Arm A or Arm B at 1:1 ratio.

Arm A (Serplulimab arm): Serplulimab + chemotherapy(Carboplatin/Cisplatin-Etoposide)+concurrent radiotherapy; Arm B (placebo arm): Placebo + chemotherapy(Carboplatin/Cisplatin-Etoposide)+concurrent radiotherapy; The 4 stratification factors for randomization include: ECOG PS (0 or 1), staging (I/II or III), radiation fraction (bid or qd), and region (Asia or non-Asia).

Inclusion Criteria:

1. Male or female, aged ≥18 years when signing the ICF.
2. Histologically diagnosed with SCLC.
3. Diagnosed with LS-SCLC (stage Ⅰ-Ⅲ of the AJCC 8th edition of the cancer staging), which can be safely treated with curative radiation doses.
4. Major organs are functioning well.

Exclusion Criteria:

1. Histologically or cytologically confirmed mixed SCLC.
2. Subjects suitable for surgery. Subjects who are suitable for surgery but refuse surgical treatment can be included.
3. Patients who have previously received systematic anti-tumor treatments for small cell lung cancer, including but not limited to radiotherapy, chemotherapy, and immunotherapy.
4. Patients with other active malignancies within 5 years or at the same time.
5. Subjects with known history of severe allergy to any monoclonal antibody.
6. Subjects with known anaphylaxis to carboplatin/cisplatin or etoposide.
7. In the judgment of the investigator, subjects who have any other factors that may lead to a premature discontinuation.